Two-electrode spark gap with interposed insulator



Dec. 5, 1967 K. J. GERMESHAUSEN ET AL TWO-ELECTRODE SPARK GAP WITHINTERPOSED INSULATOR Original Filed Dec. 27. 1960 5 Shee ts-Sheet 1INVENTORS KENNETH J. GERMESHAUSEN JOHN TURNER 1967 K. J. GERMESHAUSEN ETAL 3,356,888

TWO-ELECTRODE SPARK GAP WITH INTERPOSED INSULATOR Original Filed Dec.27, 1960 5 Sheets-Sheet 2 us l5 I 5 POWER SUPPLY ll? 47 liT STROBOSCOPEFREQUENCY Eggs? CONTROL Fig. 3.

I .INVENTORS KENNETH J GERMESHAUSEN JOHN L. TURNER 1967 K. .1.GERMESHAUSEN E AL 3,356,888

TWO-ELECTRODE SPARK GAP WITH INTERPOSED INSULATOR Original Filed Dec.27, 1960 v 3 Sheets-Sheet 5 VOLTAGE GRADIENT FACTOR INCH o l I I RADIUSOF CENTER CONDUCTOR (THOUSANDTHS OF AN INCH) Fig. 5.

INVENTORS KENNETH J GERMESHAUSEN JOHN L. TURNER United States Patent3,356,888 TWO-ELECTRODE SPARK GAP WITH INTERPOSED INSULATOR Kenneth J.Germeshausen, Weston, and John L. Turner,

Needham, Mass, assiguors to EG & G, Inc., a corporation of MassachusettsOriginal application Dec. 27, 1960, Ser. No. 78,587.

Divided and this application June 24, 1964, Ser.

7 Claims. (Cl. 313-325) The present invention relates togaseous-discharge devices, and more particularly to spark gaps. Thisapplication is a division of co-pending application Ser. No. 78,- 587,filed Dec. 27, 1960, by the applicants herein.

Applicant Germshausens co-pending application, Ser. No. 598,325, filedon July 17, 1956, now US Letters Patent No. 2,977,508 issued on Mar. 28,1961, discloses a novel gaseous-discharge device and system. The subjectinvention is useful in this and other gaseous-discharge devices tomaterially improve the performance thereof as hereinafter pointed out.

There as long been a problem of initiating regular and uniformdischarges in gaseous-discharge flashtubes. It is believed that in orderto initiate a discharge, there must be ions or free electrons availablein the discharge region which can be affected by the electric fieldproduced at the trigger electrode when the trigger impulse is applied.The electric field drives the ions or electrons through the gas towardone of the principal electrodes and the resulting collisions with gasmolecules produce ionization thereby initiating the discharge. In thepresence of ambient light, the problem of initiation is less severebecause there are phontons available to produce some photoelectronswhich will be affected by the electric field produced at the triggerelectrode. In any particular case, there may or may not be sufi'icientphotoelectrons to effect consistent breakdown, but it can be said ingeneral that the problem becomes more serious as the intensity of theambient light decreases, and in substantially total darkness the problemis greater. Because it is greatest in total darkness, we shall refer tothis phenomenon as the dark-start problem, but this term also includesthe full range of ambient light as well as complete darkness.

The dark-start problem becomes even more serious in flashtubes operatingat low average power levels, such as single flashes; irregular flashfrequencies as when the flashtube is triggered in response to the randomoccurrence of an event; or in stroboscopes or other repetitivefiashprecision applications at very low flash rates in the order of, forexample, ten flashes per second or less. In such cases, flashtubes havedemonstrated a tendency to skip, to fire late, or, at the low flashingrates, to fire irregularly or not at all.

It is, therefore, an object of the invention to provide a novelspark-gap having optimum discharge characteristics.

Another object of this invention is to provide a sparkgap for operationwithin a gaseous-discharge flashtube. In summary the present inventionconsists of a spark-gap of unique design in which a pair of electrodesare separated from each other by an insulative material across which adischarge may take place when the static breakdown potential is exceededby pulsed energy.

The invention will now be described in connection with the accompanyingdrawings FIGURES 1A and 1B of which are standard projections of aspark-discharge device, hereinafter referred to as sparker, constructedin accordance with a preferred embodiment, FIGURE 1A being a side viewpartially cut away to illustrate details of ice construction, and FIGURE1B being an end view of the right-hand end of the sparker shown inFIGURE 1A;

FIGURE 2 is a perspective view, partially cut away, of agaseous-discharge flashtube utilizing the sparker shown in FIGURE 1;

FIGURE 3 is a schematic circuit diagram illustrating a preferredelectrical system for operating the flashtube of FIGURE 2;

FIGURE 4 is a plan view, partially cut away, of the gaseous-dischargeflashtube of FIGURE 2, showing the disposition of the tube elements;

FIGURE 5 is a graph depicting the effects of variations in the design ofthe sparker.

FIGURES 1A and 1B show a sparker, indicated generally by referencedesignator 50, which consists of a cylindrical block of an electricallyinsulative material 51 such as, for example, ceramic and having a hollowcylindrical portion 52 extending the length of the ceramic block '51 andsubstantially coaxial thereto. A conductive-metal probe 53 of, forexample, tungsten, is inserted in the hollow portion 52, terminating atthe end of the block at the right-hand side shown in FIGURE 1A andsealed to and extending out of the block 51 at the left-hand end and forpurposes hereinafter indicated. A conductive metal ring 54, of, forexample, Kovar, is firmly attached to the exterior surface of the block51 at its right-hand end by brazing, crimping or the like. Attached tothe metal ring 54, as by Welding or the like, is a conductive metal lead55 Whose function will be shown below.

The sparker 50 is essentially a two-electrode spark-gap designed toproduce an electric discharge between the probe 53 and the metal ring'54 when an electric impulse is fed to the probe 53, The sparker hasbeen designed for use in a gaseous-discharge flashtube as shown in FIG-URE 2, and also to operate from the trigger impulse of the flashtube. Itis, therefore, necessary that the sparker be highly eflicient anddependable. The design must be such that a discharge is assured eachtime the flashtube is triggered. In order to make the most efficient useof the voltage of the impulse fed to the probe 53 to effect breakdown,it is essential to develop as intense an electric field as ispracticable at the surface of probe 53. Many factors are involved indesigning a spark discharge device having a very intense electric fieldadjacent to one of the electrodes, and for this reason, a theoreticaldiscussion of the subject will help to explain the advan tages of thesubject configuration.

Starting with the case of a pair of parallel-plane conductors ofinfinite extension, separated from each other by a distance d andsubjected to a voltage V, a uniform electric field is produced which maybe expressed as =V/d Where E is the electric field at each point between the conductors.

It is well known that if a coaxial configuration is substituted for theparallel-plane conductors, but maintaining the voltage and theseparation distance between the conductors the same, the electricalfield at the surface of the center conductor is more intense than thatfound in the aforesaid parallel-plane system. We have discovered thatthe intensity of the electric field at the surface of the 7 centerconductor can be further increased many fold by inserting an insulatingbody between the center and outer conductors and optimizing the designof the unit.

The six most important parameters which effect the intensity of theelectric field at the surface of the center conductor are 1) thedielectric constant (K) of the insulating material disposed between thecenter and outer conductors; (2) the radius of the center conductor (r(3) the inside radius of the insulating material (r (4) the outsideradius of the insulating material (r (5) the inside radius of the outerconductor (r.,); and (6) the voltage (V).

In order to simplify this discussion we shall make certain of theseparameters constant and we shall use the quantities of a sparkerdesigned for use with the fiashtube of FIGURE 2, as the constants. Theinsulative material should preferably be one having a high dielectricconstant in the range of, say, 8.60 to 9.50 or higher, and it should, ofcourse, have sufficient dielectric strength to withstand the electricfields produced therein. In the sparker 50, we used a high aluminaceramic (94% A1 having a dielectric constant of approximately 8.80. Ourchoice of ceramic cylinders was limited by the small variety in sizesthat were commercially available, thereby limiting our choice of r and rAccordingly, we choose a ceramic cylinder having an outside radius (r of0.020 inch and an inside radius (r of 0.005 inch.

We have discovered that with respect to the radius of the outerconductor (r.,) the most intense electric field is found at the surfaceof the center conductor when the outer conductor is sealed to the outersurface of the ceramic cylinder. If there is a spacing between the outerconductor and the ceramic, then as this spacing is increased, theintensity of the electric field at the surface of the center conductordecreases. For this reason, r., was made as close to 1' as practical.Therefore, 1' and 11, may be considered equal.

Theoretically, for such a coaxial structure of infinite lengths andperfect concentricity, the electric field E at the surface of the centerconductor is given by the following formula:

(1) E=VG where Therefore by using the value G, the voltage gradientfactor which may be defined as that factor which when multiplied by thevoltage V applied between the inner and outer conductors, results in theelectric field E at the surface of the center conductor, the effect ofvarying the radius of the center conductors r upon the voltage gradientfactor, G, can be shown independent of the voltage applied. By Formula 1the intensity of the electric field may be obtained. FIGURE 5 is such agraph.

The curve of FIGURE 5 which was plotted with r r r, and K held constantdemonstrates the variation in the voltage gradient factor G andtherefore, in the electric field E also, as the center conductor variesfrom an infinitesimally thin wire to one approaching the size of thehollow portion of the ceramic insulator r It can be seen that themaximum G is obtained when the radius 1', is a minimum. Obviously, whenthe radius r reaches zero, there is no center conductor and G is alsozero. This curve tends to show that the most intense electric field isobtained by using a center conductor with the smallest radius. This istheoretically true, but practical disadvantages sometimes make it moreadvantageous to utilize the increase in G obtained by large radiuscenter conductors as shown by the curve at the right-hand side of FIGURE5. The disadvantages involved in using extremely fine wire for thecenter conductor include the fragility of such a wire causing erosionwhich, in turn, results in a limited service life and the difficulty inhandling, assembling, and accurate and stable positioning.

These disadvantages may be less important in other applications ofsparkers but for use with fiashtubes of the type hereinafter described,practical considerations require that the up-swing of the curve at thelarger radii be used. It should be pointed out, however, that theincrease in G with the increase of r does not continue to the pointwhere r, is a maximum or r equals r It has been found experimentallythat when r and r are equal, that is, when the center probe is sealed tothe ceramic insulator, the sparker requires a very high voltage to causebreakdown (low effective G). The reason for this is easily found in thetheoretical analysis of the electric field within such a sealed,idealized structure and the electric field throughout the volume of thesparker is identical to that in the case of no ceramic at all. It shouldbe further pointed out that there may be another restriction in theapproach of r to r to effect maximizing of G on the right-hand side ofthe curve of FIGURE 5. As the gap between r, and r approachesapproximately one mean-free-path of ions and electrons in the gas underthe conditions of use, an incipient discharge may be starved for atomsor molecules to ionize, and so the idealized minimum of breakdownvoltage may not be achieved for this reason.

The curve of FIGURE 5 indicates that the most intense electric fields onthe surface of the center conductor is obtained when the radius ofcenter conductor is somewhat less than the inside radius of theinsulator. Any point, however, on this curve is a substantialimprovement over the corresponding values for coaxial systems without aceramic insulator, and parallel-plane systems. This is graphicallyillustrated in FIGURE 5 by the point A which shows a center conductor0.0035 inch has voltage gradient factor of approximately 556 per inchwhile the corresponding points for a coaxial system without an insulatoris approximately per inch (point B) and for the parallelplane system isapproximately 61 per inch (point C). The electric field indicated bypoint A is more than three times that of point B and more than ninetimes that of point C. The radius 0.0035 inch was chosen for thisexample because it is the value which we used in the sparker shown inFIGURE 1A. A center conductor with a greater radius could have beenchosen to further increase the electric field, but it was convenient forproduction purposes to use the value chosen in order to use a wire sizeidentical to that of the trigger probes, 21, 23, 25, 27 and 29 (see FIG-URE 2); and also for ease of assembly, still giving adequately lowbreakdown voltage, commensurate with tube triggering requirements andtrigger probe breakdown voltages.

The curve of FIGURE 5 is theoretical for a coaxial unit of infinitelength and perfect .concentricities and, therefore, any physicalembodiment of these principles will, of course, result in modificationsin the curve. Even with these modifications, the curve indicates therelationship between the named factors and is highly useful for thispurpose.

Although the probe 53 is shown terminating at the end of and disposed inthe center of, the ceramic block 51, such termination and dispositionare not essential and modifications thereof may be resorted to withoutdeparting from the spirit and scope of this invention.

The ceramic block 51 and the probe 53 are held in fixed relationship bysealing the probe 53 to the left end of the ceramic block as shown inFIGURE 1A, and by ring 54 at the right end which is attached to theceramic 51 and to the cathode support 17 (see FIGURE 2).

The flashtube in FIGURE 2 is shown having a glass, fused quartz ofsimilar light-transparent envelope 1 with a planar top and cylindricalside walls. This particular configuration has the advantages of maximumlight output through the planar top and minimum space required formounting the flashtube. The gaseous medium, such as exnon and the like,may be sealed within the envelope 1, as, for example, by closing off thegas-filled inlet tube 3, in the base of the envelope 1. For the purposesof variablefrequency stroboscopes and the like, it is preferable thatthe gas be maintained at a high pressure of the order of, say, one-thirdto three atmospheres, more or less. An anode electrodeS and a cathodeelectrode 7, preferably both of the same construction, are supportedspaced from one another within the envelope 1 by conductive supports 15and 17 that extend outside the base of the envelope through the bottomwall thereof. The cathode 7 and the anode are preferably both of thesintered cold-cathode type disclosed in applicant Germeshausens priorUnited States Letters Patent No. 2,492,142 issued Dec. 27, 1949, andthey are illustrated in FIGURE 2 as substantially similarrectangular-surface pills disposed substantially parallel to oneanother. Such sintered electrodes are capable of withstanding thegaseous bombardment inherent in the operation of closely spacedelectrodes at substantial voltages in a high-pressure gas.

Disposed within the space between the substantially parallel opposingsurfaces of the anode 5 and the cathode 7 are a plurality of probe-typetrigger or control electrodes 21, 23, 25, 27 and 29, of, for example,tungsten. While five such trigger electrodes are illustrated, more orless trigger electrodes may be employed consistent with the separationbetween the anode 5 and the cathode 7 and the hereinafter describedrequired discharge-conducting or guiding function of the plurality oftrigger electrodes. Trigger electrodes 21, 23, 25, 27 and 29 aresupported by and electrically attached to, by welding or the like,conductivesupport pins 121, 123, 125, 127 and 129 respectively. Thesepins enter the fiashtube through the base thereof at points spaced fromthe side walls. The trigger electrodes are attached to the pins atapproximately right angles, with the free ends of the trigger electrodesterminating in the space between the anode 5 and the cathode 7.

The free ends of trigger electrodes 23, 25 and 27 lie along a straightline L (shown dotted) between the center points at the top of the facingsurfaces of the anode 5 and the cathode 7. The free ends of triggerelectrodes 21 and 29 lie along a line (not shown) extending from the topcorners of the facing surfaces of the anode 5 and the cathode 7 on thesame side of the envelope that these trigger electrodes 21 and 29 areattached to their respective supports 121 and 129. This configuration isshown more clearly in FIGURE 4. The free ends of trigger electrodes 21and 29 are so terminated to insure that the arc breakdown between thesetrigger electrodes and the principal electrodes adjacent to each, takesplace only at the free ends of these trigger electrodes and not atdifferent places along the trigger electrodes which might be the case ifthese electrodes were extended to line L.

Sparker 50 is disposed within the gas of the fiashtube but remote fromthe discharge path between the cathode 7 and the anode 5. Sparker 50 ispositioned so that the light produced when it is energized will impinge,either immediately or by reflection, on one of the principal electrodesto assist in initiating a discharge in the fiashtube.

In accordance with the present invention, a voltage is applied betweenthe anode 5 and the cathode 7 that is of if itself insuflicient toproduce a discharge therebetween through the gas. A trigger impulse isapplied to break down the gas in the neighborhood of either the anode 5or the cathode 7 between the same and the adjacent trigger electrode 29and 21, respectively. In this embodiment, breakdown is first initiatedat the cathode 7. Simultaneously, the trigger pulse is also fed to probe53 of sparker 50 through conductive-pin support 60 causing a dischargeto take place between probe 53 and metal ring 54 which is shownconnected to the cathode support 17 by conductive wire 55. The electrodearrangement and geometry of the sparker 50, as has been pointed out, aresuch that the required breakdown potential between probe 53 and metalring 54 is less than that between cathode 7 and adjacent triggerelectrode 21 thereby insuring that a discharge will take place betweenprobe 53 and ring 54 each time the fiashtube is triggered. The lightfrom sparker 50 impinges upon the cathode 7 at the same time that thetrigger impulse is applied to adjacent electrode 21. As the light fromthe sparker 50 impinges upon the cathode 7, electrons are released fromthe cathode 7 which are subjected to the electric field produced by thetrigger impulse at trigger electrode 21. The electrons, under the forceof the electric field, collide with the gas molecules thereby producingionization of the gas and the initiation disclosed in the aforesaidco-pending application of applicant Germeshausen.

The electric circuits for operating the fiashtube of the presentinvention may assume the form illustrated in FIG- URE 3 in which a flashcapacitor or capacitors 6 is or are charged through a limiting impedance4 "from a powersupply energy source 2. The upper and lower terminals ofthe capacitor 6 are shown connected by conductors and 117, respectively,to the pins 15 and 17 connected with the anode 5 and the cathode 7. Thevoltage thus developed between the anode and cathode is, as'beforeexplained, insuflicient in and of itself to effect a discharge 1therebetween. A trigger circuit 8 may comprise a thyratron or otherswitching circuit adapted to discharge a capacitor 10 through theprimary winding 46 of a trigger transformer 45 to produce a triggerpulse. The trigger circuit 8 may be controlled by a stroboscopefrequency control 12, as of the type disclosed in United States LettersPatent No. 2,331,217 issued to applicant Germeshausen on Oct. 12, 1943.The trigger pulse so produced causes sparker 50 to fire and initiates ortriggers the successive electrode breakdown before-discussed in order topermit the energy stored in the capacitor 6 to discharge between theanode 5 and the cathode 7, thereby to produce a high intensity briefflash or repetition of flashes. Trigger circuits of this character arealso disclosed in United States Letters Patent No. 2,478,901, issued onAug. 16, 1949 to Harold E. Edgerton.

Although We have described our invention with a certain degree ofparticularity in connection with the preferred embodiment, the inventionhas a much broader scope and all changes and modifications are .deemedto fall within the spirit and scope of this invention as defined in theappended claims.

We claim:

1. A spark gap comprising, in combination:

a first electrode;

a second electrode spaced from said first electrode;

an electrically-insulative member sealed to said first electrode andspaced from said second electrode a distance greater than themean-free-path of the ions and electrons in the gas therebetween; and

means for connecting a source of electrical energy across said first andsecond electrodes.

2. A spark gap comprising, in combination:

an electrically-insulative member having an opening therethrough;

a first electrode disposed within said opening and spaced from saidmember a distance greater than the meanfree-path of the ions andelectrons in the gas therebetween;

a second electrode sealed to the external surface of said memberadjacent one end of said opening and spaced from the first electrode;and

means for connecting said first and second electrodes across a source ofelectrical impulses of suflicient potential to cause a discharge to takeplace therebetween.

3. A spark gap comprising, in combination:

an electrically-insulative member having an opening therethrough;

a first electrode disposed within said opening, spaced from said membera distance greater than the meanfree-path of the ions and electrons inthe gas therebetween and terminating adjacent one end of said member;

a second electrode sealed to the external surface of said member,adjacent said end of the member and spaced from said first electrode;and

means for connecting said first and second electrodes across a source ofelectrical impulses of sufficient potential to cause a. discharge totake place therebetween.

4. A spark gap as claimed in claim 3 and in which the said secondelectrode passes completely around said member and said first electrode.

5. A spark gap as claimed in claim 4 and in which theelectrically-insulative member, the opening in said member and the firstelectrode are all substantially concentric cylinders, and the saidmember has a high dielectric length having a cylindrically shapedopening passing therethrough from a first end to a second end thereof;

a first cylindrically shaped electrode disposed within said opening,sealed to the first end of said member and terminating adjacent thesecond end of said member;

a second cylindrically shaped electrode of a length less than saidpredetermined length, sealed to the external surface of said member andterminating adjacent the second end thereof, said first electrode beingspaced 5 from said member for a length at least equal to the I length ofsaid second electrode, said spacing being greater than themean-free-path of electrons and ions in the gas therebetween, saidsecond end of said member and the adjacent terminating first and secondelectrodes all lying substantially along a common plane; and

means for connecting said first and second electrodes across a source ofelectric impulses of sufiicient potential to cause a discharge to takeplace therebetween.

References Cited UNITED STATES PATENTS DAVID IQ GALVIN, PrimaryExaminer.

1. A SPARK GAS COMPRISING, IN COMBBINATION: A FIRST ELECTRODE; A SECONDELECTRODE SPACED FROM SAID FIRST ELECTRODE; AN ELECTRICALLY-INSULATIVEMEMBER SEALED TO SAID FIRST ELECTRODE AND SPACED FROM SAID SECONDELECTRODE A DISTANCE GREATER THAN THE MEAN-FREE-PATH OF THE IONS ANDELECTRONS IN THE GAS THEREBETWEEN; AND MEANS FOR CONNECTING A SOURCE OFELECTRICAL ENERGY ACROSS SAID FIRST AND SECOND ELECTRODES.